I have a P-channel enhancement mode MOSFET which I am trying to use for high-side power switching. (specifically, the MOSFET is the IRF9Z24N). I have an 18K resistor between gate and source and a switch from gate to 'ground'. There is a 100 ohm load resistor between drain and 'ground'. Applying a positive voltage to the source gives 'normal' operation where if the switch is open-circuit (Vg = Vs, so Vgs = 0), the drain voltage is 0 volts (FET is off), and if the switch is set to ground the gate, the drain voltage equals the source voltage (FET is on). So that's all as expected.
Where I get confused, is if no voltage is applied to the source terminal, but IS applied across the 100 ohm load instead.
By my reckoning, the Vgs is still 0 volts and the FET should be off. However, the voltage applied across the load appears at the source terminal implying that the FET must be ON !!
I've no doubt that there is a rational explanation to this, but it defeats me !!
Any advice will be welcomed !!
Where I get confused, is if no voltage is applied to the source terminal, but IS applied across the 100 ohm load instead.
By my reckoning, the Vgs is still 0 volts and the FET should be off. However, the voltage applied across the load appears at the source terminal implying that the FET must be ON !!
I've no doubt that there is a rational explanation to this, but it defeats me !!
Any advice will be welcomed !!